Magnetic resonance imaging (MRI) is a technology which uses the phenomenon of magnetic resonance to perform imaging. The principles of the phenomenon of magnetic resonance mainly include: in atomic nuclei containing a single proton, such as the hydrogen atom nuclei which are present throughout the human body, the protons have spin motion and as such resemble small magnets. Moreover, the spin axes of these small magnets have no definite regular pattern, and if an external magnetic field is applied, these small magnets will rearrange according to the magnetic force lines of the external magnetic field; specifically, they will rearrange in two directions, parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is known as the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is known as the negative longitudinal axis.
The atomic nuclei only have a longitudinal magnetization component, which has both a direction and a magnitude. In magnetic resonance imaging, a radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field, so that the spin axes of these atomic nuclei deviate from the positive longitudinal axis or negative longitudinal axis, giving rise to resonance. Once the spin axes of the above atomic nuclei have deviated from the positive longitudinal axis or negative longitudinal axis, the atomic nuclei have a transverse magnetization component.
After transmission of the RF pulse has stopped, the excited atomic nuclei emit an echo signal, releasing the absorbed energy gradually in the form of electromagnetic waves, and the phases and energy levels thereof all return to the pre-excitation state. An image can be reconstructed by subjecting the echo signal emitted by the atomic nuclei to further processing, such as spatial encoding. The process by which the above excited atomic nuclei return to the pre-excitation state is known as the relaxation process, and the time required to return to an equilibrium state is known as the relaxation time.
Since the hydrogen atomic nuclei in fat and the hydrogen atomic nuclei in water inside the human body are in different molecular environments, they have different resonance frequencies and different relaxation times when excitation is carried out using the same RF pulses. If signals are collected at different echo times, fat tissue and water display different phases and signal strengths.
Dixon methods are used to create a pure water proton image in MRI. The basic principle thereof is that two kinds of echo signals, in-phase and opposed-phase, of water and fat protons are collected separately; the two kinds of signal with different phases are subjected to an operation, each generating a pure water proton image and a pure fat proton image, thereby achieving the objective of fat suppression. There are at present many Dixon imaging methods for water/fat separation in the art, including the single-point Dixon method, two-point Dixon method and three-point Dixon method, etc.
However, with regard to any Dixon method with symmetrical acquisition (in-phase/opposed-phase acquisition), the inability to determine which is the water image and which is the fat image on the basis of the phase information thereof results in the type of water/fat image obtained by the Dixon method being unknown.
Taking the three-point Dixon method as an example, the principle thereof is to obtain an in-phase (or opposed-phase) image and two opposed-phase (or in-phase) images simultaneously, find additional phases caused by magnetic field inhomogeneity on the basis of the two opposed-phase (or in-phase) images, subject the two opposed-phase (or in-phase) images to phase correction, and then find a water image and a fat image together with the in-phase (or opposed-phase) image. In practical applications, since the additional phases found from the two opposed-phase (or in-phase) images cannot be used directly for correction of the phases thereof, the phases must be subjected to back-wrapping, while the intrinsic instability of phase back-wrapping results in the possibility of the calculated fat and water images being swapped sometimes, so that there is no way of accurately determining which is the water image and which is the fat image.
To solve this problem, Chinese patent application 201010610002.4 (inventor WENG, Dehe) discloses a method for water/fat image identification in MRI. The method includes: obtaining three echoes with no phase encoding as a reference scan; using the reference scan to calculate a reference water image projection and/or a reference fat image projection in the phase encoding direction, to obtain a reference water/fat image projection; based on a water image and fat image calculated by the three-point Dixon method, calculating a complete water image projection and/or a complete fat image projection in the phase encoding direction, to obtain a complete water/fat image projection; calculating the correlation between the reference water/fat image projection and the complete water/fat image projection, to obtain at least two correlation values; obtaining the greatest correlation value from the calculated correlation values, determining the type of the complete image projection corresponding to the greatest correlation value as the type of the reference image projection corresponding to the greatest correlation value, and determining the type of the image calculated by the three-point Dixon method on the basis of the type of the complete image projection.
In addition, those skilled in the art are still diligently searching for other solutions.